In known manner, a propeller comprises a set of blades designed to be driven in rotation by a propeller shaft. With variable pitch propellers, each of the blades is configured to be capable of pivoting about its own longitudinal axis, which axis extends in a radial direction of the propeller.
A propeller pitch control system is designed to vary the pitch of the propeller, i.e. to vary the angle formed by its blades relative to the direction of the propeller axis, which is also the direction of the propeller shaft.
One such system is described by way of example in Document U.S. Pat. No. 8,167,553, having a FIG. 2 that is reproduced herewith as FIG. 1.
That document thus presents a propeller pitch control system 10 arranged in such a manner as to control the pitch of the propeller 50 of an airplane engine.
The propeller 50 has blades 52 mounted radially around a propeller shaft 60.
The propeller shaft 60 is a hollow shaft, with its end closed by a wall 62.
In the proximity of this end, in the periphery of the shaft 60, there are provided radial openings 64 for fastening blades. A respective blade 52 is fastened in each radial opening 64.
The blades 52 are fastened in such a manner as to be capable of pivoting about their respective longitudinal axes Z (which axes are radial for the propeller 50), via ball bearings 54.
The propeller pitch control system 10 includes a blade swivel device 20 for swiveling the blades.
The device 20 includes a rotary control element constituted by a wormscrew 22 arranged on the axis X of the propeller shaft 60. The wormscrew 22 is held in position inside the shaft 60 by ball bearings 21, which position is stationary other than the possibility of rotating about its axis X.
The blade swivel device 20 also has a nut 26 that is arranged on the wormscrew 22.
The nut 26 is connected to the propeller shaft so as to rotate permanently at exactly the same speed of rotation as the propeller shaft. In this embodiment, this connection is constituted by a retaining finger 23.
In addition, the nut 26 is connected to the blade so that the axial position of the nut (along the propeller axis X) determines the angular position of the blades.
For this purpose, each blade has a blade attachment finger 56. Each of these attachment fingers 56 extends radially from the root of the blade of which it forms a part along a radial attachment axis Z′.
Each attachment axis Z′ is offset from the axis of the radial opening 64 for fastening the blade that is designed to receive the blade 52 of which the attachment finger 56 forms a part. Consequently, movement of the attachment finger 56 along the axial direction causes the blade 52 of which the finger 56 forms a part to turn about its pitch axis Z.
In order to actuate the fingers 56, the nut 26 has notches 28, with each notch 28 being designed to receive a blade attachment finger 56.
More precisely, the root of each of the blades 52 presents an off-center wrist-pin 55 that extends perpendicularly to the axis Z of the blade. The end of the wrist-pin 55 opposed to the axis Z presents a blade attachment finger 56 that projects from the wrist-pin along the radial direction Z′ towards the propeller axis X. For each blade 52, the nut 26 has a notch 28 designed to receive the blade attachment finger 56 in question. When the nut 26 moves axially along the propeller axis X under the effect of the wormscrew 22 rotating, the attachment fingers 56 all move by the same amount. Under the effect of this movement, each of the blades 52 pivots about its pivot axis Z. This pivoting movement places the blades 52 in the desired angular position.
While the propeller is rotating, and in order to ensure that the propeller pitch remains constant, the speed of rotation of the wormscrew 22 thus needs to be exactly equal to the speed of rotation of the propeller 50, so as to avoid any movement of the nut 26 along the screw 22.
Conversely, in order to change the pitch of the propeller, there must be a difference between the speeds of rotation of the propeller and of the wormscrew 22.
The direction in which the pitch of the propeller changes thus depends on the relationship between the speed of rotation of the wormscrew 22 and the speed of rotation of the propeller shaft 60.
When the wormscrew 22 is driven to rotate at a speed faster than that of the shaft 60, the nut 26 moves in a first axial direction so as to vary the pitch of the propeller in a first direction; conversely, when the wormscrew 22 is driven in rotation at a speed less than that of the shaft 60, the nut moves in a second axial direction so as to cause the pitch of the propeller 50 to vary in the opposite direction.
The wormscrew 22 associated with the nut 26 thus constitutes a blade swivel device 20 enabling the blades 52 to be placed in a desired angular position, so that they occupy a position corresponding to the desired propeller pitch. The angular position of the wormscrew (more precisely its angular position relative to the propeller shaft 60) determines the pitch angle imposed on the blades 52.
In order to actuate the blade swivel device 20, the propeller pitch control system 10 also presents a transmission 30 that is driven by a motor 40. The motor 40 is an electric motor with a rotor 42 including in particular a shaft on which a gearwheel 44 is mounted.
The transmission 30 is constituted by an epicyclic geartrain. It transmits the torque transmitted via the gearwheel 44 to the wormscrew 22 (rotary control element of the blade swivel device 20).
The transmission 30 is constituted essentially by a ring 32 having two sets of teeth 321 and 322, which ring constitutes its inlet member, and by a single planet wheel 34 that constitutes its outlet member 34. The ring 32 is supported by the propeller shaft 60 by means of two ball bearings 33. The set of teeth 321 is an outside set of teeth meshing with the gearwheel 44: the gearwheel 44 of the rotor of the motor 40 drives the ring 32 via this set of teeth 321. The set of teeth 322 is an inside set of teeth meshing with the planet wheel 34. The planet wheel 34 is supported by the shaft 60; its axis 341 thus rotates together with the shaft 60.
The end of the wormscrew 22 that is remote from the wall 62 is constituted by a gearwheel 36. The teeth of this gearwheel mesh with the teeth of the planet wheel 34.
Thus, the rotation of the rotor 42 of the motor 40 is transmitted to the wormscrew 22 by means of the transmission 30. This transmission involves a transmission ratio R, i.e. the speed of rotation of the wormscrew 22 about the axis X is equal to the speed of rotation of the ring 32 multiplied by the coefficient R.
The term “transmission ratio” is used herein to mean the ratio of the speeds of rotation respectively of the member driven by the outlet member of the transmission and of the inlet member of the transmission.
The propeller pitch control system 10 is configured in such a manner that when the inlet member 32 of the transmission 30 is driven at an appropriate speed of rotation (which is a function of the speed of rotation of the shaft 60 and of the desired pitch), the transmission 30 drives the rotor 22 of the blade swivel device 20 at a speed of rotation such that the blade swivel device 20 places the blades 52 in the desired angular position.
The propeller pitch control system 10 serves to vary the pitch of the propeller 50 and thus to modify the power demand on the engine of the airplane.
Nevertheless, that system presents a drawback.
Specifically, it requires an electric motor to operate continuously in flight. Specifically, the motor 40 is operating at all times in order to drive the rotation of the wormscrew 22 and ensure that it has a speed of rotation that is equal to the speed of rotation of the propeller shaft 60, or that is at least close to that speed. As a result of operating continuously in that way, the control system 10 presents a high level of energy consumption and it presents wear that is relatively fast.